This invention relates generally to line printers, and particularly relates to a paper guide for use in a high speed line printer.
In a commonly used high speed printing device, a character print head is transported parallel to a stationary platen with its printing elements selectively activated in a transverse direction toward the platen. The print medium is positioned between the moving print head and the platen with the impact of the moving print head on an inked ribbon in contact with the print medium thus forming the printed characters.
The print medium, which is generally paper, is provided to the high speed printer from a paper supply which generally feeds a continuous roll of paper in which individual sheets are distinguished by perforated lines. This print medium feed configuration avoids the necessity of providing individual print medium sheets to the printer. In a bi-directional line printer the print medium is generally moved by means of a lower drive mechanism located between the paper supply and the print head and an upper drive mechanism which pulls the print medium past the print head toward a print medium storage location. The print medium lower and upper drive mechanisms generally include a pair of sprockets, or cog wheels, each having a plurality of teeth around its periphery for engagement with a linear array of apertures positioned at each edge of the continuous print medium strip. In a line printer in which the print medium is moved only in one direction, only an upper drive mechanism is required to pull the print medium past the transversely moving print head.
It is important in such a configuration that the paired lower and upper drive mechanism sprockets be precisely aligned with respect to one another. If this precise alignment is absent, the print medium will be transported in a direction which is not perpendicular to the movement of the print head resulting in skewed character lines. In addition, irregularities in sprocket teeth spacing or aperture spacing on either edge of the print medium will result in non-linear character arrays or character lines angled with respect to the transverse axis of the print medium. Even with precise alignment of input and take-up sprocket assemblies, misalignment of the stationary platen along its length relative to the moving print head will result in a non-linear, irregular array of printed characters applied to the print medium by the rapidly moving print head. Still another source of irregular character arrays produced by a high speed line printer is related to environmental factors such as changes in humidity which affect physical properties of a paper printer medium, e.g., increased humidity causes paper under tension to stretch. The prior art discloses various approaches for improving line printer performance in this area.
One approach for accurately positioning the print medium along a platen relative to the moving print head is disclosed in U.S. Pat. No. 4,179,023 to DeBoo et al wherein is disclosed a paper guide which extends around the platen and includes a flexible extension which bears against the front of the moving print mechanism in guiding the paper into the small gap between the print head and the platen. The flexible final guide extends up the side of the platen adjacent the printing mechanism and bears against the printing head so as to be flexibly deflected by the print head as the print head moves across the front of the platen. The leading edge of the print medium is therefore less likely to catch on the ribbon or get caught on the edge of the paper-exit opening of the cabinet in which the printing machine is located. This invention is designed to operate with a revolving platen and relates to the input, or feeding, of the print medium to the narrow space between the print head and the platen. U.S. Pat. No. 4,242,006 to Kondur relates to an improved print medium advancement mechanism which includes a ratchet wheel mechanically coupled to print media support sprockets. The ratchet wheel provides rotational motion to a shaft coupled to the sprockets in response to an electrical signal from the printer thereby moving the print media incrementally. The print medium is held in a relatively fixed orientation relative to the revolving sprockets by means of a media guide 20 which is mechanically coupled to the disc-shaped hub 18 of sprocket 16 so that the print medium 8 is disposed between it and the cylindrical sidewall of sprocket 16. The medium guides 20 therefore ensure that the print medium will remain in contact with the sprocket drive wheels during high speed operation and that print medium movement will be regular and continuous. In addition, guides 20 also allow for the smooth tracking of the print medium in front of platen 11a in ensuring the impact of print head 13 upon a flattened print medium. Paper guides 20, however, do not compensate for misalignment of sprockets 16 relative to platen 11a and print head 13. Nor do sprockets 16 allow for improper alignment between themselves or for misalignment between sprockets 16 and the sprockets of a print medium input drive if present in the high speed printer.
Another approach to print medium adjustment in a line printer involves the use of a pulley-clutch assembly coupled to rotatable sprockets the teeth of which engage the apertures on each edge of the print medium. The push-type clutch assembly allows the serrations of the pulley to be disengaged from the pulley drive train permitting the pulley to be manually rotated in adjusting print medium tension. Once the desired tension has been applied to the print medium, the push-type clutch is then released and the rotatable sprockets coupled to pulleys which, in turn, are coupled to the pulley drive train move the print medium as required. The precision with which print medium tension is adjusted in this manner is limited by the spacing between adjacent serrations of each pulley.
The present invention is intended to avoid the aforementioned print medium misalignment problems in a high speed printer which cause the skewed orientation of character lines and non-linear character printing. These problems are avoided by means of an adjustable, precisely positioned means for accurately adjusting print medium path length between the upper and lower drive sprockets in a bi-directional printer and between the upper drive sprockets and the line defined by the translational motion of the print head in a single direction printer.